With this post I am beginning a series of posts about Apache Karaf, an OSGi container based on Equinox or Felix. The main difference to these frameworks is that it brings excellent management features with it.

Outstanding features of Karaf:

Extensible Console with Bash like completion features

ssh console

deployment of bundles and features from maven repositories

easy creation of new instances from command line

All together these features make developing server based OSGi applications almost as easy as regular java applications. Deployment and management is on a level that is much better than all applications servers I have seen till now. All this is combined with a small footprint as well of karaf as the resulting applications. In my opinion this allows a light weight development style like JEE 6 together with the flexibility of spring applications.

Exit the console. If this is the main console karaf will also be stopped.

OSGi containers preserve state after restarts

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Please note that Karaf like all osgi containers maintains it´s last state of installed and started bundles. So if something should not work anymore a restart is not sure to help. To really start fresh again stop karaf and delete the data directory or start with bin/karaf clean.

Check the logs

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Karaf is very silent. To not miss error messages always keep a tail -f data/karaf.log open !!

Tasklist - A small osgi application

Without any useful application Karaf is a nice but useless container. So let´s create our first application. The good news is that creating an OSGi application is quite easy and maven can help a lot. The difference to a normal maven project is quite small. To write the application I recommend to use Eclipse 4 with the m2eclipse plugin which is installed by default on current versions.

Features descriptor for the application that makes installing in Karaf very easy

Parent pom and general project setup

The pom.xml is of packaging bundle and the maven-bundle-plugin creates the jar with an OSGi Manifest. By default the plugin imports all packages that are imported in java files or referenced in the blueprint context.

It also exports all packages that do not contain the string impl or internal. In our case we want the model package to be imported but not the persistence.impl package. As the naming convention is usedwe need no additional configuration.

Tasklist-model

This project contains the domain model in our case it is the Task class and a TaskService interface. The model is used by both the persistence implementation and the user interface. Any user of the TaskService will only need the model. So it is never directly bound to our current implementation.

Tasklist-persistence

The very simple persistence implementation TaskServiceImpl manages tasks in a simple HashMap. The class uses the @Singleton annotation to expose the class as an blueprint bean.

The annotation @OsgiServiceProvider will expose the bean as an OSGi service and the @Properties annotation allows to add serice properties. In our case the property service.exported.interfaces we set can be used by CXF-DOSGi which we present in a later tutorial. For this tutorial the properties could also be removed.

The blueprint-maven-plugin will process the class above and automatically create the suitable blueprint xml. So this saves us from writing blueprint xml by hand.

Automatically created blueprint xml can be found in target/generated-resources

Tasklist-ui

The ui project contains a small servlet TaskServlet to display the tasklist and individual tasks. To work with the tasks the servlet needs the TaskService. We inject the TaskService by using the annotation @Inject which is able to inject any bean by type and the annotation @OsgiService which creates a blueprint reference to an OSGiSerivce of the given type.

The whole class is exposed as an OSGi service of interface java.http.Servlet with a special property alias=/tasklist. This triggers the whiteboard extender of pax web which picks up the service and exports it as a servlet at the relative url /tasklist.

Snippet of the relevant code:

Automatically created blueprint xml can be found in target/generated-resources

Tasklist-features

The last project only installs a feature descriptor to the maven repository so we can install it easily in Karaf. The descriptor defines a feature named tasklist and the bundles to be installed from the maven repository.

A feature can consist of other features that also should be installed and bundles to be installed. The bundles typically use mvn urls. This means they are loaded from the configured maven repositories or your local maven repositiory in ~/.m2/repository.

Installing the Application in Karaf

Add the features descriptor to Karaf so it is added to the available features, then Install and start the tasklist feature. After this command the tasklist application should run

Check that all bundles of tasklist are active. If not try to start them and check the log.

You can change the port by creating aa text file in "etc/org.ops4j.pax.web.cfg" with the content "org.osgi.service.http.port=8080". This will tell the HttpService to use the port 8080. Now the tasklist application should be available at http://localhost:8080/tasklist

Summary

In this tutorial we have installed Karaf and learned some commands. Then we created a small OSGi application that shows servlets, OSGi services, blueprint and the whiteboard pattern.

In the next tutorial we take a look at using Apache Camel and Apache CXF on OSGi.

Karaf uses your local maven to resolve the feature. So you have to make sure that karaf knows about your maven on startup. So make sure your MAVEN_HOME is set correctly. Best thing is to use the same shell to mvn install the takslist project and start karaf.